1. Field
The invention relates to pressurized sterilizers, and more particularly to means for venting the chamber of such a sterilizer.
2. State of the Art
Apparatus for sterilization by pressurized vapor in a pressure chamber is well known. Both dry articles and liquids can be sterilized in such chambers. After a pressure cycle is complete, the chamber must be depressurized or vented to facilitate opening of the chamber door for removal of the load and to prevent injury to the operator. It is desirable to vent the load as quickly as possible so that the goods are available more quickly and sterilizing cycles can be performed more rapidly. However, where the load comprises sterilized liquids, a too-rapid drop in chamber pressure will cause some of the liquid to boil off and be lost. Losses of more than about 5% of liquid volume are considered unacceptable. Thus, it is important that the rate of pressure drop during venting of a liquid load remain below the rate at which significant liquid boil-off occurs.
One previous method of venting is to open a bleed valve having a relatively small orifice, so that the pressure drops at a slow rate. Venting in this manner is rapid at first, but slows considerably as the pressure drops, so that the venting time is relatively long.
An improvement described in U.S. Pat. No. 4,781,898 to Jones, provides two vent paths. The first vent path is a slow bleed valve Which directs the steam into a condensing coil. The second path is activated when the chamber pressure has dropped below a predetermined level, and vents rapidly and directly to the atmosphere. However, while provision of the second path speeds up the venting process, it does not control the rate of venting in the early stage. Moreover, this two-channel venting requires extra plumbing and space and is accordingly more expensive.
Another venting method is described in U.S. Pat. No. 4,971,764 to Albright. In this method, an orifice having a relatively large flow coefficient is used. A sterilizer control governs opening and closing of the orifice according to duty cycles based on the detected rate of pressure and/or temperature drop within the chamber, thereby varying the effective flow coefficient. The duty cycle is repeatedly re-calculated to keep the rate of venting constant. However, the implementation of this method is relatively complicated and expensive. Errors can arise in calculation of the duty cycles. Also, the control of the duty cycle in progress is not responsive to the rate of pressure drop occurring during that duty cycle.
Consequently, a need remains for simple means to vent a liquid load at a controlled rate which keeps liquid loss within a desired range, while still providing a rapid vent cycle.